Blitzed306 wrote: So poly for control arms seems like a bad idea. Rear differential bushings seem like a good place for poly. Are their any companies that offer complete bushing kits that are rubber? Or high quality poly?
I'll reiterate: https://www.flyinmiata.com/performance-rubber-bushing-kt.html
revrico wrote: In reply to Blitzed306: Mazdasport competition and IL Motorsports both offer 30-40% stiffer than stock rubber. I'm presently hunting for OEM replacement rubber, and so far the only place that sells it that I've found won't let me check the price because I don't have an event timesheet or racing organization membership card. I have tracked down OEM part numbers though, so I'll get around to checking the dealership this week. Which leads to a new question because I'm not this car inclined: Would the rear control arms do better with rubber or poly? Without steering, I'm guessing that means they only function on one plane so the risk of binding isn't as high as with the front? From what I'm taking away, front/rear sway bar bushings and differential bushings would be fine in poly, control arms in rubber? If money wasn't a problem for me right now, this would be over and done with with a order to FM for rubber bushings and frame rails, but I'm still reeling from my tax hit this year and trying to save my pennies for the ecotek swap.
For OE rubber (Original Equipment, not OEM aka Original Equipment Manufacturer, that's something different) go to the various places that sell factory parts for less. Med Center Mazda, for example. Or probably even Mazda Competition, they offer stock parts at killer prices. I suspect that's the one place you found.
The front wheels don't depend on the bushings to allow for steering. That's what ball joints are for. So poly bushings are no more or no less suitable for the front than for the rear.
Sway bar bushings do okay in poly, and they're a whole lot easier to lubricate if required. Diff mounts don't move, so poly is 100% suitable. Might be too much NVH for your personal preference, but that's a durometer question as opposed to material.
In reply to Keith Tanner:
yea, it was mazda competition. Thanks for helping me figure this whole mess out.
I'll have to ask around at the next autocross, I'm willing to bet at least one of the NAs there has poly bushings by now, so going for a ride and getting a feel for them shouldn't be too far fetched.
This is looking like my most in depth personal maintenance to date, I swear changing the transmission in my old work truck was simpler, on paper. I also think a lot of what's causing over thinking is how much debate there is on the topic. I'm now in the camp that stock did extremely well for 23 years and 130k miles, so stock should be a fine replacement with no track use and maybe 12 autocrosses a year. They're just starting to look worn out, so I feel like they should be replaced.
revrico wrote: Which leads to a new question because I'm not this car inclined: Would the rear control arms do better with rubber or poly? Without steering, I'm guessing that means they only function on one plane so the risk of binding isn't as high as with the front?
The question of "plane" is not related to steering, it's related to the alignment cams.
Think about how the A-arms move, starting with the rears because it's simpler. They are nominally perpendicular to the chassis -- if that were actually the case then you could use solid metal hinges on them and it would work fine. The reality is that they are not perpendicular, because you want a small amount of static toe in. To do this, the Miata has alignment cams in the inner lower A-arm pivot points, which allows you to move those pivot points inboard or outboard by half an inch or so each way. To give it a small amount of toe in you move the front cam inboard and the rear cam outboard, which angles the lower A-arm forward by half a degree or so. Now if you draw a line through the pivot points, that line is no longer parallel to the centerline of the chassis, but offset by that same half a degree. However, since the same line drawn through the center of the chassis mounting points is perpendicular, the space in between the pivot bolts and the A-arm mounting sleeves (the space taken up by the bushing) changes in shape as the arm moves. With rubber bushings this isn't a problem, because rubber is squishy. Poly is also squishy, just less so. Delrin (a hard plastic) is not squishy at all, and is totally unsuitable to using in those kinds of mounts. If you try to use it, then the non-squishy plastic will basically prevent the arm from moving very far ("bind") and produce insane and unpredictable spring rates.
The fronts are the same idea, except that it's caster that drives the need to have the bushings misaligned, because toe is taken care of by the ball joints.
There are 22 A-arm bushings on a Miata, 4 each for the front front corners (2 inners on upper & lower arms) and 7 each for the rear corners (2 inner, 2 outer on the lower arm, 2 inner 1 outer on the upper arm). Of those 22 bushings, 8 of them are alignment cams (the lower inner ones one each corner).
As was mentioned before, poly has its own binding problems, but they're related to lubrication rather than the angles and planes of the alignment cams.
codrus wrote: The question of "plane" is not related to steering, it's related to the alignment cams.
This is where my problem was with picturing it. I was thinking the control arms just worked vertically, forgetting about alignment.
I'll be the very first to admit I am very uneducated on this whole topic. I can replace big parts, I can usually tell when they're broken. That's where it ends for me. But I'm trying to learn. And this forum, more than any other I belong to or have belonged to, is extremely good at teaching even just by watching.
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